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More New Trapdoor Spiders

December 26, 2011: Introduction: The Brush-footed Spider family Barychelidae is represented worldwide by 44 genera and 303 species (Platnick 2011). Thirteen of these genera have only a single pair of spinnerets (Raven 1994; Dippenaar-Schoeman 2002).


Another important generic character of the barychelids is the size of the paired claws, of the 13 two spinneret barychelid genera, there are only three genera having paired tarsal claws I and II very reduced compared to claws on legs III and IV viz., Diplothele O. P.-Cambridge 1890, Synothele Simon, 1908 and Tigidia Simon, 1892 (Raven 1985). Of these, only Diplothele represented by three species was previously known from India (Siliwal et al. 2009; Platnick 2011). The most closely related genus to Diplothele is considered to be Tigidia.


The genus Tigidia was thought to be endemic to Madagascar and Mauritius Islands where it is represented by eight species (Platnick 2011). All the species of this genus were described a century ago and are known only from their type locality. The genus was established with the description of T. mauriciana by Simon (1892) from Mauritius. Tigidia remained monotypic until Benoit (1965) added two genera, Forsythula Pocock, 1903 and Tructicus Strand, 1907, to its synonymy. Further, Raven (1985) also synonymised three more genera (Acropholius Simon, 1902; Cestotrema Simon, 1902; Nossibea Strand, 1907) from Madagascar with Tigidia.


The Western Ghats is known for its rich and endemic fauna being a biodiversity hotspot (Myers et al. 2000). However, the present knowledge of its invertebrate fauna is meager and the region likely supports a wealth of invertebrate fauna which is still unknown (Daniels 2003; Mirza & Sanap 2010). During surveys in the central Western Ghats of Karnataka, authors (NG and MS) collected a barychelid that had two spinnerets. Initially, it was considered to belong to the Indian genus Diplothele. Diplothele and Tigidia are very close allied genera but Raven (1985) listed two distinct characters to distinguish between them, namely, ocular area wider behind than in front and the presence of preening comb on metatarsi (Image 1). On examination of the specimen under the stereomicroscope it was found that the species had a preening comb and an ocular area wider at the back than in front, indicating the specimens collected from Uttara Kannada belong to the genus Tigidia. Later, two more species were collected from the southern Western Ghats of Tamil Nadu by RS and ZM, which were distinctly different from the specimens from Karnataka. It is possible that this genus occurs throughout the Western Ghats but remained unnoticed due to its vertical trapdoor burrows.


Based on the new distribution pattern Tigidia is probably a Gondwanan relict. Morphologically, the closest genus to Tigidia is an African genus Pisenor, which has a similar combination of characters than the Indian genera Diplothele and Tigidia but differs in that Pisenor retains the putatively plesiomorphic character that all paired claws of the legs are the same size; in Tigidia and Diplothele, the paired claws of legs I and II are very small, relatively about half the size of the claws on legs III, IV (Table 1). The 13 barychelid genera with two spinnerets are known from Australia, South Asia and the African subcontinent (Raven 1985; Platnick 2011) seem to be an ideal group to study evolutionary lineage and also to test the Gondwana theory (Datta-Roy & Karanth 2009; Kunte in press). Based on the present finding, the following hypotheses are proposed: (i) Pisenor is the sister genus of Diplothele and Tigidia; (ii) The genus Tigidia evolved after the Indo-Madagascar plate separated from Africa 160 million years ago; and (iii) Diplothele evolved between 50 to 80 million years ago after the Indian plate separated from Madagascar and collided with the Eurasian plate. Phylogenetic studies will be carried out on Tigidia and other closely allied genera to test and justify the aforesaid theories and hypothesis and those will be published separately. Download the complete report in PDF format here.


Source: Manju Siliwal, Neha Gupta, Rajesh V. Sanap, Zeeshan Ali Mirza & Robert Raven Threatened Taxa.


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